Power devices are widely used to carry large currents and support high voltages. For example, circuits in motor drives, appliance controls, robotics, lighting ballasts and other applications often require semiconductor switching devices that can carry large currents and support high blocking voltages. One type of power device is the bipolar junction transistor (BJT). The bipolar junction transistor has been the switching device of choice for many high power applications because of its ability to handle relatively large current densities and support relatively high blocking voltages.
A BJT typically includes a semiconductor material having two opposing p-n junctions in close proximity to one another. Thus, BJTs may be referred to as “n-p-n” or “p-n-p” transistors. In operation, charge carriers enter a region of the semiconductor material of a first conductivity type adjacent one of the p-n junctions, which is called the emitter. Most of the charge carriers exit the device from a region of the semiconductor material of the first conductivity type adjacent the other p-n junction, which is called the collector. A third region of the semiconductor material, known as the base, is positioned between the collector and the emitter and has a conductivity type that is opposite the conductivity type of the collector and the emitter. The two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter.
BJTs are current controlled devices in that a BJT is turned “on” (i.e., it is biased so that current flows from the emitter to the collector) by flowing a current through the base of the transistor. When current is injected into or extracted from the base, depending upon whether the BJT is n-p-n or p-n-p, the flow of charge carriers, i.e., electrons or holes, which can move from the emitter to the collector, may be affected. By flowing a small current through the base of a BJT, a proportionally larger current passes from the emitter to the collector. Typically, a BJT may require a relatively large base current (e.g., one fifth to one tenth of the collector current) to maintain the device in its “on” state. As high power BJTs have large collector currents, they also typically have significant base current demands. Relatively complex external drive circuits may be required to supply the relatively large base currents that can be required by high power BJTs. These drive circuits are used to selectively provide a current to the base of the BJT that switches the transistor between its “on” and “off” states. Structural and operational details of BJTs are discussed in Solid State Electronic Devices by B. Streetman (2nd edition (1980), chapter 7).
The material that makes up a device can contribute to the operability and usefulness of the device. For example, conventional BJTs are typically formed of silicon (Si), but can also include gallium arsenide (GaAs) and indium phosphide (InP). Silicon carbide (SiC) has also been used as a material for BJTs. SiC has potentially advantageous semiconductor characteristics, for example, a wide bandgap, high electric field breakdown strength, high thermal conductivity, high melting point and high-saturated electron drift velocity. Thus, relative to devices formed in other semiconductor materials, for example, Si, electronic devices formed in SiC may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under high radiation densities. SiC BJTs are discussed, for example, in U.S. Pat. No. 4,945,394 to Palmour et al., and U.S. Pat. No. 6,218,254 to Singh et al.
Due to device properties such as relatively low on-resistance at relatively high current density, positive temperature coefficient (PTC) for the on-resistance, and/or relatively fast switching speeds, SiC power bipolar junction transistors (BJTs) may be desirable for use in high-power systems. SiC BJTs may have the potential to operate at high temperatures and/or relatively harsh environments, for example, due to the absence of a gate oxide. However, SiC BJTs typically require a continuous base drive current. Also, while SiC BJTs may provide a relatively high current gain, the open base breakdown voltage may be significantly reduced. The current gain of SiC BJTs may also be limited by recombination in the base, the base-emitter space charge region, and/or surface recombination.